Although silver halide emulsions are employed in both photographic imaging and radiographic imaging, these imaging applications are in fact quite dissimilar.
In photography diffuse electromagnetic radiation within or near the visible spectrum is topically reflected from a subject gathered by a lens to expose a silver halide emulsion imaging unit coated on one side of a support.
In radiography X-radiation from an essentially point source is passed through a subject. The object is to record areally variations in the intensity of the X-radiation penetrating the subject. Ideally the image is formed with just that component of the X-radiation that is not scattered during subject penetration. To assist in accomplishing this objective, X-radiation penetrating the subject is commonly passed through a grid which is capable of transmitting a much higher proportion of unscattered X-radiation than scattered X-radiation. The X-radiation pattern is passed to the radiographic element from the grid. No lens is employed in radiographic imaging.
Silver halide radiographic elements actually exhibit relatively low levels of sensitivity to X-radiation, since most of the X-radiation passes through the silver halide grains and only a minor portion is absorbed. Two approaches, neither of which have a counterpart in photography, are commonly used in combination to increase the imaging speed of radiographic elements. First, the absorption efficiency of the radiographic element can be doubled by using a "dual coated" format in which silver halide emulsion layer units are coated on opposite sides of the film support of the radiographic element. The second approach is to mount an intensifying screen adjacent each silver halide emulsion layer unit. The intensifying screen typically consists of a particulate phosphor and binder coated on a support. The phosphor particles absorb X-radiation much more efficiently than silver halide and promptly emit longer wavelength electromagnetic radiation, typically light, which the silver halide emulsion layer unit can absorb more efficiently. A dual coated radiographic element mounted between a front and back pair of intensifying screens typically exhibits an imaging sensitivity about an order of magnitude higher than that of the radiographic element used alone.
Since dual coated radiographic elements divide the image information between the emulsion layer units on opposite sides of the support, the support of the dual coated radiographic element is necessarily transparent to permit transmission viewing of the superimposed images. This leads to the problem of loss of image sharpness due to crossover. Crossover occurs when an intensifying screen exposes not only the adjacent emulsion layer unit, but the emulsion layer unit coated on the opposite side of the support as well.
Abbott et al U.S. Pat. Nos. 4,425,425 and 4,425,426 recognized that the use of spectrally sensitized tabular grain silver halide emulsions offered the capability of dramatically reducing crossover. When spectrally sensitized tabular grain emulsions are compared to emulsions containing spectrally sensitized nontabular grains at the same silver coating coverages, tabular grain emulsions offer dramatic crossover reduction advantages.
Dickerson et al U.S. Pat. Nos. 4,803,150 and 4,900,652 taught the formation of "zero crossover" dual coated radiographic elements by adding to the dual coated radiographic element structures of Abbott et al, cited above, the additional feature of processing solution bleachable crossover reducing dye layers coated between each of the emulsion layer units and the film support. Since the technique used to measure crossover exposure cannot separate the small increment of exposure produced by direct absorption of X-radiation within the emulsion layer units from crossover exposure, "zero crossover" radiographic elements are understood to extend to those that exhibit measured crossover levels of less than 5 percent.
With so many potential sources of image degradation in radiographic imaging that have no counterpart in photography it is not surprising that the analysis of image quality in radiographic elements has evolved differently. The historical and still predominant approach to comparing image quality is to rely on visual inspection and ranking by a trained observer, such as a radiologist. Through side-by-side comparisons of subject exposures, a trained observer can offer an informed opinion of which exposure is offering more imaging information.
A second standard by which the imaging qualities of radiographic elements are compared is detective quantum efficiency (also referred to as DQE). DQE is simply a measure of input noise divided by output noise. Since output noise is a combination of input noise and the increment of noise imparted by the radiographic element, DQE is typically much less than unity (1.0).
From 1937 until the 1950's the Eastman Kodak Company sold a dual coated (Duplitized.TM.) radiographic film product under the name No-Screen X-Ray Code 5133. Since the product was intended to be exposed directly by X-radiation rather than by an intensifying screen, the grains were not spectrally sensitized. The tabular grains accounted for greater than 50% of the total grain projected area while nontabular grains accounted for greater than 25% of the total grain projected area. Based on remakes of the emulsion it was concluded that the tabular grains had a mean diameter of 2.5 .mu.m, an average tabular grain thickness of 0.36 .mu.m, an average aspect ratio of 7:1, and an average tabularity (defined below) of 19.2. The product which superseded Code 5133 contained essentially nontabular grains.
It was not until after the discovery by Abbott et al, cited above, of reduced crossover in dual coated radiographic products being realized by use of spectrally sensitized tabular grain emulsions that tabular grain emulsions exhibiting high tabularity were introduced into radiographic products Tabular grain emulsions are those in which &gt;50% of the total grain projected area is accounted for tabular grains. Tabular grain emulsions of high tabularity are those that satisfy the relationship: EQU D/t.sup.2 &gt;25
where
D is the equivalent circular diameter (ECD) in micrometers of the tabular grains and PA1 t is the thickness in micrometers of the tabular grains.
High tabularity tabular grain emulsions have also been investigated extensively for use in photographic elements for reasons that are totally unrelated to crossover reduction. A variety of photographic advantages that have no applicability to radiography have been identified, such as increased blue to minus blue speed separations and increased sharpness with the incorporation of tabular grains in selected layers of a multilayer format of interest in color photography. A few advantages, such as increased covering power, are applicable to both radiography and some forms of photography.
In view of their divergent exposure requirements it is not surprising that particular modifications of high tabularity tabular grain emulsions intended to optimize performance for a particular photographic application can be detrimental to radiographic utility and vice versa. In addition to the differences in exposure requirements, radiographic elements and photographic elements often require incompatible processing. Radiographic elements are, for example, generally required to be fully processable in less than 90 seconds. This places an upper limit on iodide concentrations in radiographic elements that are well below optimum iodide levels for most color photography requirements.
A number of photographic applications are recognized to be benefitted by having the highest attainable levels of grain uniformity. A photographic concern from the outset of investigations related to high tabularity tabular grain emulsions has been the polydispersity of the grains. In the earliest tabular grain photographic emulsions dispersity concerns were largely focused on the presence of significant populations of nonconforming grain shapes among the tabular grains conforming to an aim grain structure. FIG. 1 is a photomicrograph of an early high tabularity silver bromoiodide emulsion first presented by Wilgus et al U.S. Pat. No. 4,434,226 to demonstrate the variety of grains that can be present. While it is apparent that the majority of the total grain projected area is accounted for by tabular grains, such as grain 101, nonconforming grains are also present. The grain 103 illustrates a nontabular grain. The grain 105 illustrates a fine grain. The grain 107 illustrates a nominally tabular grain of nonconforming thickness. Rods, not shown in FIG. 1, also constitute a common nonconforming grain population in tabular grain silver bromide and bromoiodide emulsions.
While the presence of nonconforming grain shapes in tabular grain emulsions has continued to detract from achieving narrow grain dispersities, as procedures for preparing tabular grains have been improved to reduce the inadvertent inclusion of nonconforming grain shapes, interest has increased in reducing the dispersity of the tabular grains. Only a casual inspection of FIG. 1 is required to realize that the tabular grains sought themselves exhibit a wide range of equivalent circular diameters.
A technique for quantifying grain dispersity that has been applied to both nontabular and tabular grain emulsions is to obtain a statistically significant sampling of the individual grain projected areas, calculate the corresponding ECD of each grain, determine the standard deviation of the grain ECDs, divide the standard deviation of the grain population by the mean ECD of the grains sampled and multiply by 100 to obtain the coefficient of variation (COV) of the grain population as a percentage. While very highly monodisperse (COV&lt;10 percent) emulsions containing regular nontabular grains can be obtained, even the most carefully controlled precipitations of tabular grain emulsions have rarely achieved a COV of less than 20 percent. Research Disclosure, Vol. 232, August 1983, Item 23212 (Mignot French Patent 2,534,036, corresponding) discloses the preparation of silver bromide tabular grain emulsions with COVs ranging down to 15. Research Disclosure is published by Kenneth Mason Publications, Ltd., Dudley Annex, 21a North Street, Emsworth, Hampshire P010 7DQ, England.
Saitou et al U.S. Pat. No. 4,797,354 reports in Example 9 a COV of 11.1 percent; however, this number is not comparable to that reported by Mignot. Saitou et al is reporting only the COV within a selected tabular grain population. Excluded from these COV calculations is the nonconforming grain population within the emulsion, which, of course, is the grain population that has the maximum impact on increasing grain dispersity and overall COV. When the total grain populations of the Saitou et al emulsions are sampled, significantly increased COVs result. In a remake of the Example 9 emulsion of Saitou et al a COV of 21.3 percent was observed when COV was based on the total grain population.